Semiconductor lasers, as coherent light sources, have found a wide range of applications including optical communications, barcode readers, laser pointers, CD/DVD/Blu-Ray disc reading and recording, laser printing, and laser scanning. Such lasers are fabricated on a semiconductor wafer (e.g. GaAs or InP) with a limited diameter (typically 2-6 inches). With increased wafer size or reduced chip size, each wafer can produce more laser chips, resulting in reduced cost per chip. While the wafer size of a semiconductor laser has been increased from 1 or 2 inches to 4 or 6 inches, the size of the semiconductor laser chip itself has remained the same to meet output power requirements.
A majority of semiconductor lasers used in optical communications are edge emitting lasers, with a resonant structure formed by a straight waveguide between two mirrors (cleaved facets). As the current density in the active region is limited by material property, the total injection current cannot be larger than a certain amount. At the same time, the quantum efficiency of the semiconductor laser is limited, and hence the slope efficiency of the laser is also limited. Therefore, the length of the semiconductor laser waveguide cavity must be larger than a length threshold to meet the output power requirements.
Semiconductor distributed feedback (DFB) lasers are widely deployed in optical network units (ONUs), with passive optical networks having a typical dimension of a few hundred micrometers in length and width. While the length of a semiconductor laser is dictated by its cavity length, its width has to be larger than a certain size to accommodate the bonding pad (typically 100 μm×100 μm), as well as to maintain a desired spacing between the bonding pad and the laser waveguide structure. As wafer growth and processing costs for semiconductor lasers are more or less fixed, the cost of the semiconductor laser is largely determined by the number of chips that can be produced per wafer. Given a fixed wafer size, reducing the size of the semiconductor laser chip can bring significant cost benefit.